1. Field of the Invention
The present invention relates to an optical system for scanning by optical beams for electrophotography or the like in which scanning with optical beams are made to form an image.
2. Description of the Related Art
Heretofore, an optical beam scanning apparatus is used as an image writing device in the electro-photographic process and is mounted in a laser printer, a laser facsimile or the like which is an output unit of a computer and a facsimile. Recently, in order to further improve the high-speed operation and the high resolution, a demand for the scanning apparatus using a plurality of optical beams is being increased.
FIG. 7 is a schematic diagram illustrating a conventional multi-optical beam scanning apparatus for making scanning with two optical beams.
In FIG. 7, the conventional multi-optical beam scanning apparatus includes first and second optical sources 1a and 1b for emitting optical beams, shaping lenses 2 for shaping the optical beams from the optical sources 1a and 1b, a beam splitter 3 for directing the optical beams emitted from the shaping lenses 2 toward a predetermined position, a deflector 4 for deflecting the optical beams from the beam splitter 3, a scanninglens 5 for focusing the optical beams deflected by the deflector 4 on a surface 12 to be scanned, a synchronization detector 60 for detecting synchronization of the optical scanning beams, light receiving sensors 7a and 7b for receiving optical beams for the reference of the detection of synchronization by the synchronization detector 60, slits 8 formed in front of the light receiving sensors 7a and 7b, a housing 9 for accommodating above-mentioned constituents of the apparatus, and a mirror 13 for reflecting write optical beams from the scanning lens 5 through a window 17 of the housing 9 onto the surface 12 to be scanned. The optical beams of the optical sources 1a and 1b are subjected to the modulation control by a control circuit 14, and a processing circuit 150 computes to detect the synchronization of a write timing from a signal from the synchronization detector 60 to control the control circuit 14.
In the prior art configuration, since the light receiving sensors 7a and 7b are arranged or juxtaposed in a perpendicular direction (sub-scanning direction), as shown in FIG. 7, to the scanning direction (main scanning direction) of the plurality of optical beams, detection positions of the synchronization may be slightly varied and different depending on mounting of the light receiving sensors 7a and 7b and it is difficult to pertinently position the light receiving sensors with respect to the optical beams slightly separated from each other in the sub-scanning direction. Accordingly, a writing position by scanning of each of optical beams 10 and 11 tends to be deviated due to such variations in the synchronization detection positions, so that there is the possibility that fluctuation is produced in a formed image and the image is deteriorated remarkably.
In order to cope with the above problem, it is necessary to finely adjust write timings for two optical beams in each multi-optical beam scanning apparatus provided in the subsequent stage to the synchronization detector 60 and the light receiving sensors are required to be mounted with high accuracy.
Particularly, since the space between pixels in the sub-scanning direction is narrowed as the resolution is increased and for example, for 600 dpi, the space is as small as about 42 .mu.m. Accordingly, the space between a plurality of scanning beams is also narrow and there is a problem that the mounting accuracy of the light receiving sensors 7a and 7b in the sub-scanning direction is high so that the mounting is difficult and adjacent optical beams tend to be detected in error.